Conventionally, a CRT has been used widely as a display unit for displaying characters and pictures by exciting phosphors using electron beams. Generally, in a phosphor screen formed on an inner face of a glass panel of the CRT, three kinds of phosphor pixels that emit red, green, and blue lights respectively are arranged regularly as dots or in a stripe shape via a photoabsorption film referred to as a black matrix.
Such a phosphor screen can be obtained by: forming a photosensitive resin film on an inner face of a glass panel of a CRT; forming phosphor-formation cites at positions where phosphor pixels are formed on the photosensitive resin film by applying, exposing, and developing a photoreactive substance using a photolithographic technique; subsequently applying a phosphor suspension on the inner face of the glass panel; and forming respective phosphors of blue, green, and red by repeating the same photolithographic technique.
As an application process for forming a phosphor screen of a CRT, a method of applying a phosphor slurry prepared by suspending phosphors in photosensitive resin while a glass panel is rotated with a tilt is mainly employed. The processes described below are carried out by a continuous looped machine sequentially, a mill machine operated in a circular manner, or the like.
As a first step, a phosphor slurry is injected onto an inner face of a glass panel rotating at a low speed. While the injected phosphor slurry is spread over the inner face slowly due to the inclination and rotation of the glass panel, phosphor particles are precipitated (an application process). In a process of applying phosphors, it is important to obtain a phosphor screen with a uniform thickness and without unevenness in an application condition. For that purpose, some methods such as a method of changing a tilt angle of a glass panel periodically by synchronizing with a rotation period of the glass panel (for example, Publication of Japanese Unexamined Patent Application No. Hei 3-122944) and a method of rotating a glass panel by positive rotation and reverse rotation (for instance, Publication of Japanese Unexamined Patent Application No. Hei 5-101775) are proposed.
As a subsequent step of the application process, excess phosphor slurry is shaken off by increasing the rotation speed of the glass panel (a shake-off process). In order to obtain a uniform phosphor screen, setting of a tilt angle and a rotation speed of the glass panel are important in shaking-off the excess phosphor slurry. A method of shaking-off with a glass panel being oblique and facing upward (for example, Publication of Japanese Unexamined Patent Application No. Sho 55-57230), a method of shaking-off with a glass panel being oblique and facing downward (for example, Publication of Japanese Unexamined Patent Application No. Sho 59-186230), and the like have been proposed. Excess phosphor slurry is recovered in an external fluid-recovery pan that is provided beside a glass panel head in a high-speed shake-off process or is recovered in corner cups that are positioned at the four corners of a glass panel and are provided on a stage fixed to the glass panel in a swivel head part.
After shaking off the excess phosphor slurry, the phosphor screen is dried by an infrared heater from the outside (a drying process). Then a shadow mask is set, and the phosphor screen is exposed with ultraviolet rays. A light cross-linking reaction between photosensitive resin and a sensitizing initiator progresses by the irradiation of the ultraviolet rays, thus making the exposed parts insoluble in water. After the exposure, the shadow mask is removed, and development is carried out using a warm water shower or the like. As a result, unexposed parts are washed away by the water and phosphor patterns are formed only in necessary parts.
Recently, a display for a CMT is required to have a high luminance and high resolution over the entire part of a display screen on a glass panel. For this purpose, some methods, for example, a method of making a high luminance and high contrast compatible by providing a filter having the same color as respective color at the color-formation sites in a phosphor screen and combining with a high-transmission glass panel, and a method of improving reflectance by controlling a pigment concentration of phosphors having pigments that are coated with the same color minute pigment particles on the phosphors themselves used for forming a phosphor screen have been proposed.
As a method of forming a phosphor screen, there is a method of improving luminous efficiency by using phosphor particles with a large particle size. On the contrary, there is a method of obtaining a higher luminance by filling minute phosphors particles at high density. When phosphor particles with a large particle size are used, a drying method by low-speed rotation is employed in order to avoid the occurrence of a so-called cross phenomenon (nonuniformity in thickness due to the influence of a base) of phosphors during the formation of a phosphor screen. On the other hand, when minute phosphor particles are used, considering efficient recovery of the phosphor particles, a drying method by middle- to high-speed rotation is employed (for example, Publication of Japanese Unexamined Patent Application No. Hei 3-230451).
In a method of recovering excess phosphor slurry in corner cups provided at the four corners of a glass panel as described above, in the case of using phosphors having a large particle size beyond 5 .mu.m, when the rotation speed of the glass panel is too low, the centrifugal force decreases. As a result, in a drying process in forming a phosphor screen, the recovered phosphor slurry spatters from the corner cups to the outside. Therefore, in order to restrain the spatter, the glass panel requires to be rotated at high speed. However, the high-speed rotation causes the above-mentioned cross phenomenon.
On the contrary, in order to avoid the cross phenomenon, it is necessary to make the rotation speed of the glass panel as low as possible. However, when decreasing the rotation speed, the recovered phosphor slurry spatters from the corner cups to the outside. Therefore the surroundings get dirty, thus inducing defects in partially-processed articles. As described above, there are conflicting requirements as to the rotation speed of the glass panel. Consequently, there has been a problem that phosphors with a large particle size cannot be used for forming a phosphor screen to obtain a higher luminance.
From another point of view, as a method of forming a phosphor screen employing a drying process by a low-speed rotation, there is a method of recovering excess phosphor slurry in a recovery pan provided outside a head by driving each head intermittently. However, there have been problems such as great increase in the size of equipment and the complexity of a system controlling each process.